This invention relates to a novel method for photoexposing a coated sheet in a vacuum printing frame prior to etching. The novel method may be used, for example, in preparing apertured masks for use in color television tubes and display picture tubes. The invention includes a novel printing plate which is especially useful in the novel method.
The preparation of apertured masks by photoexposure and etching has been described previously; for example, in U.S. Pat. No. 3,199,430 to S. A. Brown, U.S. Pat. No. 3,313,225 to N. B. Mears, U.S. Pat. No. 3,757,250 to J. J. Moscony et al. and U.S. Pat. No. 4,061,529 to A. Goldman et al. In a typical process, a thin sheet of metal, such as cold-rolled steel or a copper-nickel alloy, is coated on both major surfaces with a light-sensitive acid-resist or enamel. Then, the coated sheet is positioned between two photomasks or printing plates, each plate carrying an opaque photographic master pattern on its inward-facing surface, each master pattern being accurately positioned with respect to the other. Each plate is usually comprised of a central area bearing the photographic master pattern and a peripheral area which is clear. The glass plates are mounted in spaced relation from one another in an equipment referred to as a vacuum printing frame. When the coated sheet is in the desired position, the printing frame is evacuated whereby the glass plates are pressed together so that the master patterns are held firmly against the coatings on the sheet. Then, the coatings are exposed to actinic radiation which passes through the glass plates for a time interval until the coatings are suitably exposed. The frame is then devacuated; that is, brought back to atmospheric pressure. The glass plates are separated and the metal sheet with the exposed coatings thereon is removed from the frame. The exposed coatings may now be developed by removing the more-soluble portions thereof. Then, the sheet may be selectively etched, and finally the less-soluble portions of the coatings are removed from the sheet.
One of the problems encountered previously is the relatively poor resolution of the optical image produced during the exposure through the master pattern. Each master pattern is comprised of opaque particles in a binder, such as silver particles in gelatin. The poor resolution is attributed to the relatively large thickness of the pattern which produces a penumbra at the edges thereof, and to variations in thickness of the opaque areas, particularly the tapered thicknesses at the edges thereof, which exhibit partial transmission of the exposing radiation.
Because of poor resolution, and for additional reasons, it is desirable to employ a master pattern of a metal or a metal oxide which is thinner, is substantially uniformly thick and is substantially uniformly opaque to the exposing radiation. Masters of this type which are used to make semiconductor devices have opaque areas that are too thin, and therefore are too transmitting for the longer, more intense exposures used to make apertured masks for cathode-ray tubes. It has also been found that masters of this type can have opaque areas that are too thick, resulting in substantial losses due to cracking and peeling of the opaque areas of the pattern.
Another of the problems encountered previously in making apertured masks for cathode-ray tubes is the relatively long time period required for evacuating and devacuating the printing frame during production operations. Where the photographic master comprises a raised pattern of light-absorbing particles in a binder, such as silver-metal particles or carbon particles in gelatin, part or all of the coating on the peripheral areas of the plate may be cut, etched or sandblasted away to speed the evacuation and devacuation processes. With these different expedients, evacuation times of 45 to 150 seconds are required for each exposure. Where the master pattern is an array of metal or metal oxide areas, the master pattern is substantially flush with the surface of the glass plate, and the time periods required for evacuating and devacuating the frame are substantially longer. When shorter time periods for evacuation and devacuation can be realized, the output from the frame can be increased.
It is desirable, for making apertured masks, to realize the potentially-better image resolution possible with a metal or metal-oxide photographic master pattern. It is economically necessary, when using a metal or metal-oxide master pattern in a vacuum printing frame, to realize the shorter evacuation and devacuation times possible with a raised pattern.